CHAP. XIX 
THE OCHIL AND SIDLAW ERUPTIONS 
303 
two flows, lying still lower in the system, project into the sea. One of 
these presents a section of much interest. It shows a central solid portion, 
jointed into rudely prismatic blocks, with an indefinite platy structure, which 
gives it a roughly -bedded aspect. Its upper ten or twelve feet are sharply 
marked off by their slaggy structure, ending upwards in a wavy surface like 
that of the Vesuviaii lava of 1858. Into its fissures, steam -cavities and 
irregular hollows, fine sand has been washed from above, as at lied Head, 
while immediately above it comes a coarse conglomerate of the u.sual character 
(Fig. 76). Still lower 
down, beneath some 900 
feet of remarkably coarse 
conglomerate, another 
group of sheets of an- 
desite abuts at Crawton 
upon the coast, with 
which, at a short dis- 
tance inland, it runs 
parallel for more than 
two miles, coming back 
to the sea at Thornyhive 
Bay and at Maidenkaim. 
We have then to pass 
over about 5000 feet of 
similar conglomerates, 
until, after having crossed 
several intercalated sheets of andesite, we meet with the last and lowest of 
the whole volcanic series of this region in the form of some bands of porphyrite 
at the Bellman’s Head, Stonehaven. The peciuliar geographical conditions that 
led to the formation of the coarse conglomerates appear to have been established 
at the same time that the volcanic eruptions began, for as we descend in 
the long coast section, we find tliat the coarse sediment and the intercalated 
lavas cease on the same general horizon. Below that platform lie some 
5000 feet of red .sandstones and red shales, yet the base of the series is 
not seen, for the lowest visible strata have been faulted against the schists 
of the Highlands. It is thus obvious that more than 5000 feet of sediment 
had been laid down over this part of the floor of Lake Caledonia before the 
first lavas were here erupted. 
2. The Sidlatv and Ocliil Grouj) 
The volcanoes which poured out the masses of material that now form 
the chain of the Ochil and Sidlaw Hills appear to have been among the most 
vigorous in the whole region of Lake Caledonia. Their chief vents 
probably lay towards the south-west in the neighbourhood of Stirling, where 
the lavas, agglomerates and tuffs discharged from them reach a thickness of 
not less than 6500 feet, without revealing their bottom. From that centre 
